Major applications of printed matters having an optically diffractive image transferred thereon by the method for image formation and intermediate transfer recording medium according to the present invention include, for example, output of images and output of a photograph of the face or the like onto identification, passports, credit cards, ID cards, or other cards.
An intermediate transfer recording medium comprising a transfer layer having both an optically diffractive image, such as a hologram or a diffractive grating, and a thermally transferable image is intended to improve security for forgery prevention, because the intermediate transfer recording medium can render, together with a thermally transferred image, unique decorative images and three-dimensional images, can realize a high level of design, and, at the same time, these holograms and diffractive gratings cannot be easily produced due to the necessity of a high level of production techniques. However, it should be noted that the application of the present invention is not particularly limited to those only so far as the applications require a high level of design and security.
There has been an increasing demand for the formation of both an optical diffraction structure and a thermally transferred image on a desired object (an object on which a structure or an image is to be transferred). To meet this demand, for example, the following method has been proposed.
Specifically, an intermediate transfer recording medium comprising a transfer layer provided separably on a base material is provided. The transfer layer comprises at least a peel layer, a relief formed layer having an optically diffractive, a transparent thin film layer having a refractive index different from the relief formed layer, and an image receptive layer. A colorant such as a dye or a pigment is transferred onto the surface of the image receptive layer using a thermal transfer sheet comprising a coloring material layer containing a heat-fusion ink or a dye to form a thermally transferred image. Thereafter, the intermediate transfer recording medium is heated to transfer the optically diffractive image and the thermally transferred image onto an object.
The image formed by this method is not present on the outermost surface and is in such a state that the image is protected by the peel layer, the relief formed layer and the like. In images used under severe conditions such as ID cards, however, a further improvement, for example, in fastness to abrasion, light, and plasticizer has been desired.
The present applicant has disclosed a method for image formation that can produce a thermally transferred image possessing excellent various fastness properties. In this method, an intermediate transfer recording medium comprising a transfer part, in which a hologram image has been set on at least every other image plane, is provided. An image is formed on the transfer part, and the transfer part is then transferred onto an object. Thereafter, a next transfer part in the intermediate transfer recording medium is again transferred at least once onto the object with the image formed thereon (see, for example, patent document 1).
Further, in order to prevent the occurrence of cracking or discoloration of a transparent thin film layer by heat and stress applied in the transfer of a transfer layer in an intermediate transfer recording medium onto an object, or a deterioration in surface appearance as a result of the appearance of a pattern due to cracking in the transparent thin film layer conformed to heat deformation of a base material in the whole transfer by a heat roller or the like, the present applicant has further disclosed an intermediate transfer recording medium with a hologram, comprising a base material and at least a peel layer formed of a heat-resistant transparent resin having high heat resistance, a relief formed layer, and a metal thin film layer stacked in that order on the base material (see, for example, patent document 2).
According to the conventional methods, the occurrence of cracking, discoloration, and cracking-derived pattern in the metal thin film layer can be prevented, and, thus, it is expected that the problem of deteriorated surface appearance is overcome. Since, however, the peel layer having a heat resistant surface is exposed, adhesion in the second transfer as described in patent document 1 onto the surface of the peel layer or in the stacking of other material onto the surface of the peel layer is disadvantageously poor.
In order to prevent the deterioration in surface appearance of the transparent thin film layer caused by heat applied in the transfer, a method for image formation and an intermediate transfer recording medium have been desired in which, even when the second transfer is carried out on the layer with the heat resistant peel layer formed thereon, the adhesion is maintained and, in addition, the protection of the thermally transferred image by a layer formed by the second transfer can provide an image formed object having both an optical diffraction structure and a thermally transferred image printed thereon in which the thermally transferred image possesses fastness properties high enough to withstand use under severe service conditions.
[Patent document 1] Japanese Patent Laid-Open No. 254840/2002
[Patent document 2] Japanese Patent Laid-Open No. 361622/2004